1. Field of the Invention
This invention relates to a method for controlling biofouling in a variety of applications including water treatment, pulp and paper manufacture and oil field water flooding. More specifically, this invention relates to a method for controlling biofouling with a combination of an antifungal or antibiotic and a chelator.
2. Description of Related Art
Biological fouling on surfaces is a serious economic problem in many commercial and industrial aqueous process and water handling systems. For example, in 1993 North American companies spent $1.2 billion on water treatment chemicals alone to fight corrosion and fouling caused by microbial organisms embedded in biofilm attached to the surfaces of pipelines. Fouling comprises a biomass which is the buildup of microorganisms and/or extracellular substances, as well as dirt or debris that become trapped in the biomass. Bacteria, fungi, yeasts, diatoms and protozoa are only some of the organisms which cause buildup of a biomass. If not controlled, the biofouling caused by these organisms can interfere with process operations, lower the efficiency of processes, waste energy and reduce product quality.
Cooling water systems used in power-generating plants, refineries, chemical plants, air conditioning systems and other commercial and industrial operations frequently encounter biofilm problems. This is because cooling water systems are commonly contaminated with airborne organisms entrained by air/water contact in cooling towers, as well as waterborne organisms from the systems' makeup water supply. The water in such systems is generally an excellent growth medium for these organisms. If not controlled, the biofilm biofouling resulting from such growth can plug towers, block pipelines and coat heat transfer surfaces with layers of slime, and thereby prevent proper operation and reduce equipment efficiency. Furthermore, significant increases in frictional resistance to the flow of fluids through conduits affected by biofouling results in higher energy requirements to pump these fluids. In secondary oil recovery, which involves water flooding of the oil-containing formation, biofilms can plug the oil-bearing formation.
Perhaps most significantly from an economic point of view, it has recently been demonstrated that biofilms adhering to stainless steel and other metal pipeline surfaces can shift the open circuit potential of the metal, thereby accelerating the propagation rate of corrosion. Although biofilms can contain any type of microorganism, including algae, fungi and both aerobic and anaerobic bacteria, these films are often comprised of sulfate-reducing bacteria which grow anaerobically in water, frequently in the presence of oil and natural gases. Colonies that include several kinds of bacteria and fungi can form deposits on metal surfaces, building slime layers and producing organic acids that cause pitting and accelerate corrosion of pipelines and associated metal structures. Replacing corrosion-damaged pipelines and related industrial infrastructure each year represents a serious drain on the nation's, and indeed the world's economic output.
Currently used methods of controlling biofouling fall generally into two categories: chemical and abrasive. Of these methods, chemical controls are generally considered to be the most effective, both in performance and cost. However, the efficacy of chemicals where biofilms are concerned is limited by the natural defense mechanisms of the embedded microorganisms. Planktonic or free-floating organisms are readily destroyed by many chemical agents used to control microorganisms. But sessile, or fixed organisms located on pipeline surfaces, are protected by a polysaccharide covering, or glycocalyx, and will have some success in warding off the effect of even fairly toxic biocides. An increased dose of toxin may or may not succeed in overcoming the protection provided by this polysaccharide covering, because these polymers restrict permeability of the biofilm by most biocides.
A wide variety of biocides that are capable of killing planktonic microorganisms are cited in the literature; see, for example, U.S. Pat. No. 4,297,224. They include the oxidizing biocides: chlorine, bromine, chlorine dioxide, chloroisocyanurates and halogen-containing hydantoins. They also include the non-oxidizing biocides: quaternary ammonium compounds, isothiazolones, aldehydes, parabens and organo-sulfur compounds. Traditionally, the above biocides have been employed to kill planktonic microorganisms in circulating water systems such as, for example, chemical refinery cooling systems or industrial pasteurizers. Until relatively recently, little routine monitoring of biocidal efficacy versus sessile microorganisms had been performed. Studies have confirmed that many widely used biocides are relatively ineffective against sessile microorganisms; see, for example, Costerton (1988). As noted above, abrasive methods of biofouling control can also be used. These methods include simple manual removal of slime, cleaning with high pressure water streams, use of cleaning "pigs" or other methods making use of a longitudinally inserted shaft, and sand blasting. To illustrate some of the disadvantages of abrasive cleaning, consider the following technique for cleaning the interior of pipes and tubing by a device that comprises a flexible longitudinal shaft with one end connected to a circular brush and the other end connected to a motor that rotates the shaft for turning the brush. The motor is generally electrically or air driven. The device is inserted within the tube or pipe to be cleaned, and herein lies the first problem: the tubes and pipes to be cleaned are limited in length to the shaft length. In this method, the maximum pipe length is limited by the friction of the trailing flex shaft/tube casing on the inside of the pipe. The minimum tubing diameter size is approximately 3/4 inch due to the required size of the flex shaft and case. Another problem is that the device is inoperable around bends of 90 degrees. Yet an additional problem is that the trailing flex-shaft and casing are very difficult to clean and maintain in a clean state under use. Also, this device is expensive to operate since it requires power such as electricity and/or shop air to run the motors in addition to, preferably, a pressurized water or cleaning solution. Other disadvantages of this and various other abrasive cleaning methods include (i) the need for protection of non-metallic surfaces such as expansion joints and valve seals, (ii) the extensive piping systems which are required for water jet cleaning, (iii) the labor-intensive nature of these methods, and (iv) the necessity of removing spent abrasive with methods such as sand blasting.
Clearly, a need exists for an effective, low toxicity method of removing and preventing water system biofouling which overcomes the disadvantages of currently known and implemented chemical and abrasive cleaning methods.